The aim of this reseach is to investigate the possible role of cyclic and highly phosphorylated nucleotides in controlling cell proliferation in rat liver. We have recently found two previously unreported nucleotides in acid soluble extracts of rat liver which carry a higher negative charge than ATP or GTP; both nucleotides contain modified adenosine as the nucleoside. One of these, which has one less phosphate than the other, seems to appear preferentially in regenerating liver. The full identity and metabolic roles of these two nucleotides are currently under study. We have developed a novel prcedure involving in vivo labeling with 32pi for determination of only the actively turning over fractions of four cyclic nucleotides (cyclic-AMP, cyclic-GMP, cyclic-CMP and cyclic-UMP) in rat liver. The concentrations of cyclic-AMP and cyclic-GMP thus obtained are in good agreement with published data. A drawback for in vivo studies is the difficulty of achieving adequate labeling of the cyclic nucleotides due to the large endogenous pools of inorganic phosphate, necessitating very large doses of 32Pi. We plan to extend the procedure to studies of liver cell cultures where this problem can be readily circumvented.